VINYLPHOSPHONIC ACID-MODIFIED CALCIUM ALUMINATE AND CALCIUM SILICATE CEMENTS

Cementitious materials in terms of calcium phosphate cements (CPC) wer e prepared through the acid-base reaction between vinylphosphonic acid (VPA) and calcium aluminate cement (CAC) reactants or calcium silicat e cement (CSC) reactants at 25 degrees C. Using CAC, two factors were responsible for the development of strength in the cements: one is the formation of an amorphous calcium-complexed vinylphosphonate (CCVP) s alt phase as the reaction product, and the other was the high exotherm ic reaction energy. Because the formation of CCVP depletes the calcium in the CAC reactants, Al2O3 . xH(2)O gel was precipitated as a by-pro duct. CCVP --> amorphous calcium pyrophosphate hydrate (CPPH) and Al2O 3 . xH(2)O --> gamma-AlOOH phase transitions occurred in the CPC body autoclaved at 100 degrees C. Increasing the temperature to 200 degrees C promoted the transformation of CPPH into crystalline hydroxyapatite (HOAp). In the VPA-CSC system, the strong alkalinity of CSC reactant with its high CaO content served in forming the CPPH reaction product which led to a quick setting of the CPC at 25 degrees C. Hydrothermal treatment at 100 degrees C resulted in the CPPH --> HOAp phase transit ion, which was completed at 300 degrees C for both the VPA-CAC and VPA -CSC systems, and also precipitated the silica gel as by-product. Alth ough the porosity of the specimens was one of the important factors go verning the improvement of strength, a moderately mixed phase of amorp hous CPPH and crystalline HOAp as the matrix layers contributed signif icantly to strengthening of the CPC specimens.